Wild type strains of Herpesvirus saimiri are highly oncogenic in various species of New World primates. Two non-oncogenic variants have been independently derived by laboratory manipulation in cell culture. A deletion of DNA sequences has occurred independently in the same region of the genome in the generation of both attenuated strains. The proposed research will: 1) definitively determined whether the deletion is indeed responsible for the loss of oncogenicity of one of these strains (llatt) and 2) characterize in detail the RNA and protein products derived from this region of DNA. A region of 1.5 kilobase pairs (kb) has been deleted from strain 11 DNA in the generation of llatt. Cloned DNA fragments will be prepared from within and spanning the 1.5 kb sequence. Permissive owl monkey kidney cells will be co-transfected with llatt DNA and a cloned DNA fragment spanning the 1.5 kb sequence (marker rescue). Recombinant virus that has re-acquired the 1.5 kb sequence will be tested for oncogenicity in marmosets. The nature of the RNA and protein products from this region will be determined using S1 nuclease mapping, hybrid selected translation, immunoprecipitation, and DNA sequencing. The eventual goal is to derive the complete sequence for this segment of DNA and its presumed protein product. In this manner, we hope to understand the role of this region of the genome in tumor formation and the lack of oncogenicity of the attenuated strains.